TYI-S-33 (and a variant BI-S-33) devised for the axenic cultivation of Entamoeba histolytica and other Entamoeba has proved, during 17 months of use, to be clearly superior to TP-S-1 medium in supporting the growth of a variety of Entamoeba species. The results of studies conducted with 33 patients, Hospital General, CMN, I.M.S.S. Mexico City, diagnosed as having hepatic amebic disease suggests that the human host, insofar as the management of iron is concerned, responds to amebic infection as it does to bacterial and fungal infections. Cecal lesions have been produced with axenically cultivated E. histolytica for the first time in an experimental animal (newborn guinea pig). Giardia lamblia can be serially cultivated axenically in either TP-S-1 or TYI-S-33 medium providing these media are sterilized by filtration. No growth is obtained in autoclaved media. On the otherhand, E. histolytica will grow in these media regardless of the method of sterilization. However, growth is at a different rate.